Semiconductor image sensors are used to sense radiation, such as lights. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) are widely used in various applications, such as digital cameras or mobile phone cameras. These devices utilize an array of pixels located in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate, and convert the sensed radiation into electrical signals.
Typically, an image sensor includes a pixel region and a periphery region. The pixel region includes a plurality of pixels. The periphery region includes circuits. Isolations for each region are required. However, different devices need different depths of trenches. A conventional dual shallow trench isolation (STI) process presents a problem in that an STI oxide step height is different between the pixel region and the periphery region. The oxide step height causes profiles of polysilicon related items to not match between the pixel region and the periphery region. Non-matching of the polysilicon related items results in poor electrical properties.
In modern semiconductor fabrication, it is common to have several regions with different isolation pattern densities in a die. However, different isolation pattern density results in severe micro loading effects. As such, the micro loading effects may cause an isolation step height difference between an isolated region and a dense region. The oxide step height causes profiles of polysilicon related items to not match between the isolated region and the dense region.